Pulse communication system



June M. G. CROSBY v PULSE COMMUNICATION SYSTEM 2 Sheet-Sheet 1 Filed April 30, 1941 n'nnn nnnn-nnnn INVENTOR Mil? 62011 1;

ATTORNEY -the steady unmodulated carrier Patented June 4, 1946 UNITED STATE PULSE COMBIUNICATION SYSTEM Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 30, 1941, Serial No. 391,059

18 Claims.

, This invention relates to improvements in radio communication over long distances, and particularly toward reducing the effects of multipath transmission in communication systems.

It has been known in the radio art that long distance radio transmission is accompanied by the arrival of signal power at the receiver over more than one path. The multiple paths are produced because the waves which reach the receiver have been refracted and reflected between the ionosphere and the earth a number of times, because of refraction from different ionized layers or regions in the atmosphere and because of magneto ionic splitting of the waves into components of different velocities. There have also been many instances of transmission simultaneously around the earth in different directions. A common result of the multipath transmission between transmitter and receiver is that useful signal modulations arriving at the receiver are badly distorted and do not faithfully reproduce the modulations of the transmitter. It often happens that the differenc in phase relations between the signals arriving over several paths produce complete fade-out.

It is known to transmit keyed signals by keying a steady unmodulated carrier (continuous wave) or to key a carrier which is modulated by a sinusoidal or approximately sinusoidal wave (interrupted continuous wave). Where interrupted continuous waves have been employed, it has been the practice to apply the modulation as an amplitude, phase or frequency modulation, and to key the wave in the usual make and break manner. It has been found that both of these known types of keyed signals are subject to fading due to the multipath phenomenon, and that (continuous wave) is affected more adversely by multipath fading than the interrupted continuous wave.

The primary object of the invention is to minimize or substantially reduce the efiects of fading and signal distortion caused by the multipath phenomenon. To achieve this object, it is proposed to transmit signals by means of pulses which are so short that when received over multiple paths the resulting succession of pulses do not overlap. Putting it another way, it is proposed to transmit the pulses with spacing between adjacent pulses greater than the delay time between the first and last significant or important pulses received over the multiple paths for each transmitted pulse. Thus, it will be evident that by means of the present invention there is obviated the possibility of th energy arriving at a receiver over one of the multiple paths cancelling that arriving over another path, and there is thus removed the underlying caus of fading due to the multipath phenomenon.

A more detailed description of the invention follows in conjunction with drawings, wherein:

Figs. 1 and 2 are graphs given to aid in an understanding of the present invention:

Figs. 3 and 4 show schematically, transmitter circuits for transmitting spaced pulses in accordance with the principles of the present invention; and

Figs. 5a and 5b are graphs given to aid in an understanding of the operation of the system of Fig. 4.

Referring to Fig. there are shown a plurality of groups of pulses I, 2, 3, 4, etc., each group of pulses representing the pulses received at a receiving station over a plurality of paths as a result of a single pulse transmitted at a remote station. By referring to each group of pulses of Fig. 1, it will be noted that each group is made up of a series of pulses constituted by an initial pulse a followed by later arriving pulses b and c. The relative intensities of the pulses in each group vary erratically, depending upon the conditions of the paths over which they arrive at the receiver. And, although the first pulse a is shown to be of greater amplitude than the succeeding pulses b and c, it should be understood that in practice the first arriving pulse of each group may be of smaller amplitude than a later arriving pulse. Each pulse group of Fig. 1 comprising the pulses a, b and c is representative of a series of pulses which may be received at a receiver when a single transmitted pulse is transmitted in accordance with the graph of Fig. 2. It is proposed, in accordance with the invention, tospace the transmitted pulses at the transmitter as shown in Fig. 2) so that the time t between succeeding transmitted pulses is greater than the time delay between the first and last significant important pulses (here shown as a and 0) received over the multipaths for each.

transmitted pulse. As an illustration, the impulses sent out at thetransmitter may have a frequency of recurrence of 1000 per second, in which case the time t shown in Fig. 1 would be ,5 of a second. This illustration of 1000 impulses per second is not to be construed as a limitation, since obviously the recurring pulses may have different frequencies so long as the essential limitation concerning the spacing between impulses, as mentioned above, is followed.- This rate of recurrence of the impulse-s transmitted often depends upon multipath conditions existing at the time of transmission, and also upon the'frequency of the transmitted carrier.

Fig. 3 illustrates one method by which suitably spaced impulses can be transmitted in order to achieve the primary object of the present invention. Referring to this figure, there is shown a transmitter circuit composed of a high frequency source of carrier waves 1 which feeds into a suitable modulator 8, the output of which is coupled to a power amplifier 9 for radiating the transmitted pulses from an antenna t0. keying device II is shown coupled to the power amplifier 9 for keying the radiated power in the usual make and break fashion; This'keyingdevice may be replaced by any known system of am- A suitableplitude modulation for applying'tone or similar modulation to the power amplifier 9.

In order to interrupt the carrier wave, and pro duce suitably spaced pulses, there i provided an impulse generator I in the form of a" blockin oscillator of'a typewell kri'o' wn'iii the-art for obtaming impulses in televisionsysterns; This'im pulse generator is shown asan electron discharge device whose anode is coupled to one winding of a transformer t3; the other wi iidin-gof which is shown coupled tothe control grid through a condenser i2 and a variable grid leak 2, thus providing 'aiii-ridllc'tivefeedback oscillator. The Value or the "grid leak 2 determines the rate of recurrence of 'tne'imptnses' generated by'the blocking oscillator I. Only the essential elements of thi oscillator'have been shown, s'inceit-forms no part of" the present invention-per se. Output energyfrom the impulse generator i is taken from the potentiometer 3--- in series with one windingof the transformer [-3 and is fed through a high-frequency pass circui't consisting of a condenser 4 and a resistor 5 to the input oi'anampl-ifier' 5. high pass circuit 4, 5 filters out the audio oscillations which may be produced in the blocking oscillator circult, and preventsthese from being-passed on to the amplifier 6. The impulses passing through the circuit 4, 5' which are impressed upon the amplifier 6' occur at the start and stop of the square wave form dot produced in the blocking oscillator I and may tak the-formof an exnemeiy sharp pulse; Amplifier 6 serves to augment the intensityof the sharp impulses inrpressed upon it to a level sufficient to completely modulate energy-from carrier 1 fed into modulator 8. Modulator 8 may be a grid-modulated stage chosen so that no radio frequency energy passes therethrough until an impulse arrives from impulse generator'l and amplifierfi. v That is, a high bias may be applied to the grid" of the modulator so that no radio frequency energy is passed by the modulator until the," grid thereof is biased positive or to its working point by the volt-- age from theamplifier 6. The radio frequency output radiated from antenna I0 is represented by the spaced impulses of radio frequency energy, as shown in Fig. 2. In referring to Fig. 2-, the first four impulses represent a dot, while the last eight impulses represent a dash, the intervening time between the dot and the dash representing a space. I

Fig. 4 illustrates a system for applying frequency modulation to a pulse generator wherein the carrier is steady in frequency and the radiated energy is chopped into short pulses by means of an amplitude modulator. In this system, the frequency modulation is applied to the ulse generator so that the number of pulses per second is modulated in accordance with the modlaws 50! to the modulator 502.

4 ulation. That is, in the case of telegraph keying, a mark might produce 1000 pulses per second, while a space might produce 800 pulses per sec end. The pulse generator of Fig, 4 comprises, in effect, a pair of dual-triode electron discharge devices 504 and 505, each of which is arranged in a multivibrator circuit in accordance with thesystemdescribed in my pending application Serial No; 273360, filedMay 13', 1939', now United States Patent 2,269,417, to which reference is made. A condenser C1 and a resistor R1 in circuit with certain electrodes of the dual triodes 504 (as shown in the drawings) control the frequency of oscillation of this particular vacuum tube. Similarly, condenser C2 and resistor R2 in circuit with certain electrodes ofdual-triode 505 control the frequency of oscillation of this last electron discharge device. In both devices the oscillation frequency is determined by the time constants or values of the particular condensers C1, C2 and resistors R1, R2. Actually, thetwo time constant circuits C1, B1 and C 2 R2 are tunedtodifferent frequencies so that if each dual-mode isexoited separately different frequencies of oscillation would'be olotainedin their common output represented by the plate resistor R: :Beca'use the two tubes are coupled togethe'nthrough thev common plate resistor R, they are-locked-ir'r step and the frequency of" oscillation when both are working simultaneously mid way betweenthe individual frequencies ror the 'unmodu'lalted condition. Modulation is applied to the grids of the two tube structures not directly associated with the time constant elements; in order thatth'erelative amplitudesofoscillation may be varied; In this way, the frequency of the output is modulated between the' individual frequencies" of the two vacuum tube circuits.

The dual-triodes 504 and 505 of Fig. 4- constitute, in effect, agene'rator of'squarewave pulses whose wave form i's-repr'esented-in Fig; 5a. The square Waves" of Fig. 5a produced by the pulse generator 504 and 505 are impressed upon high frequency passcircuit C5; Re" which converts the wave form of Fig. 5a to thewave form of Fig. 5b, prior to passing the energy on to the amplifier 506-. effect, the high frequencypass circuit C3, Ril'may' be considered a differentiating circuit which gives an output proportional to the" rate of change of the appliedvoltage; in rder to produce pulses as shown in Fig. 5b: Either the positive or negative impulses of Fig. 5b maybe used to affect the modulator 502'" after being amplified'in 506. Amplifier 506 serves' to raise the level of the waveform of the energy applied thereto, as

represented in Fig. 5b, to a level suflicient to properly modulate the energy applied, by appa- V I Apparatus 50f represents a stable 'sourceof carrier frequency, which, if desired,- may be followed by frequency multipliers and amplitude limiters. The" output from the modulator 502 is amplified in a power amplifier 503" before being radiated over an anten'na 501 in the form or spaced radio frequency pulses with a variable recurrence rate; From what has been given above; itwillbe noted that therelative amplitudes of" the two pulse generators 504 and 505 are modulated by the signal and the rate of pulse generation" is modulated'between the two pulse rates of the individual generators.

If the'modulation applied 'to' Fig. 4' is telegraph keying; the keying voltage is fed" to the" coil of relay '508. Switches S1 and S'zare'throwntojth'e' left 'so as to-connect the right-hand grids tore sistors 509 and H], and to the relay contacts. The arm of the relay 508 is connected to battery 50'! so that when contact is madeto one of the relay points, the particular right-hand grid to which it is connected is biased by the battery 501,. This unbalances the relative amplitude of oscillation so that the frequency of the squarewave-form dots is shifted towards the circuit of the tube with the strongest oscillation amplitude.

If tone or similar modulation is to be applied, the signal is applied tothe primary or transformer 5| l. The secondary of this transformer differentially modulates the two right-hand grids so that their relative amplitudes of oscillation are varied. For this type of modulation, switches S1 and 52 are thrown to the right to connect the grids to the transformer.

Condenser C3 and resistor R3 form the differentiating circuit which converts the square-wave form of Fig. 5a to the impulses of Fig. 5b. The duration of the pulse may be adjusted to be very short by adjusting the bias on amplifier 506 and modulator 502 so that only the peaks of the impulse operate the modulator. For longer pulses, the biases are adjusted so that more of the pulse is effective.

Reception of this type of pulse transmission, where the pulse rate is frequency modulated, is accomplished with an amplitude modulation receiver which feeds a frequency modulation subcarrier receiver. The output of the amplitude modulation receiver consists of a variable fre-.

quency which has the frequency of the pulse rate. This variable frequency tone is fed to a frequency modulation receiver which consists of a limiter and a means for converting the variable-frequency subcarrier into an amplitude modulated subcarrier. The amplitude modulated subcarrier is detected in the normal manner, The output of the subcarrier receiver thus corresponds to the frequency variations applied to the pulse rate.

The impulse transmitter of Figs. 3 and 4 is advantageous from the standpoint of transmitter economy. Since the impulse is only present for a fraction of the time that a steady carrier would be, the power drawn by the power amplifier is reduced and the tube heating is also reduced. This allows a heavier loading of the power tubes and a consequently higher tube output.

- What is claimed is:

1. The method of reducing fading in a radio circuit produced by multipath effects in the atmospheric signal medium between a transmitter and receiver which comprises radiating signal energy in pulses having an audio frequency rate of recurrence, each pulse having .time duration which is less than the delay time between the first and last significant or important pulses received over the multiple paths for each radiated pulse, and modulating the number of pulses per second prior to radiation in accordance with the signals to be transmitted.

2. The method of reducing fading in a'radio circuit produced by multipath effects in the atmospheric signal medium between a transmitter and a receiver which comprises producing squarewave form pulses, modulating the amplitude of the pulses in accordance with signals to thereby produce a corresponding modulation of the pulse rate, obtaining sharp pulses from the slopes of said amplitude modulated square wave form pulses, modulating a stable carrier frequency by said sharp pulses to produce interrupted radio frequency pulses having a spacing therebetween whichis greater than the delay time between the first and last significant or important pulses received over the multiple paths for each radiated radio frequency pulse.

3. In combination, a square-wave form oscillator comprising a" pair of electrode structures, each structure having an anode, a cathode and a grid; a frequency determining circuit coupled to the anode of one structure and the grid of the other structure; a common cathode resistor for both said structures; an output circuit coupled to the anode of said one structure; signal modulating means coupled to the grid of said one structure; said output circuit. comprising elements 30 arranged as to transform the squarewave pulses of said oscillator into shorter pulses which are proportional to the rate of change of the square wave applied to said output circuit;

' a source of carrier radio frequencyoscillations;

and a modulator for modulating said source by said transformed pulses to produce interrupted radio frequency pulses. V l

4. A system for reducing fading in radio circuits caused by the multipath phenomenon in the signal medium comprising, in combination, a square-wave form audio frequency oscillator constituted by a pair of electrode structures, each structure having an anode, a cathode and a grid; a frequency determining circuit coupled to the anode of one structure and the grid of the other structure; a common cathode resistorfor' both said structures; an output circuit coupled to the anode of said one structure; signal modulating means coupled to the grid of said one structure;

said output circuit comprising elements so arranged as to transform the square wave pulses of said oscillator into shorter pulses which are proportional to the rate of change of the square Wave applied to said output circuit; a stable source of carrier radio frequency oscillations; and a modulator for modulating said source by said transformed pulses to produce interrupted radio frequency pulses, the spacing between transformed pulses of the same polarity being greater than the delay time between first and last significant or important pulses received over the multiple paths for each transmitted radio frequency pulse.

5. In combination, a square wave form oscillator comprising a pair of electrode structures, each structure having an anode, a cathode and a grid; a frequency determining circuit coupled to the anode of one structure and the grid of the other structure; a common cathode resistor for both said structures; another similar square wave form oscillator having a, different frequency than said first oscillator; signal modulating means coupled to the grid of said one structure of both oscillators; a common output circuit coupled to the anode of said one structure of both oscillators, said output circuit comprising elements so arranged as to transform the square wave pulses of said oscillators into pulses which are proportional to the rate of changeof the square wave applied to said output circuit, a stable source of carrier radio frequency oscillations; and a modulator for modulating said source by said transformed pulses to produce interrupted radio frequency pulses.

6. A transmitter system for reducing fading in radio circuits caused by the multipath phenomenon in the atmospheric signal medium comprising a source of high frequency Waves, a modulator coupled to said source, an impulse generator coupled to said modulator for interrupting said carrier wave, an amplifier inthe output of said modulator, an antenna coupled to said amplifier, and an amplitude modulation system coupled to said amplifier, said impulse generator including a diiferentiator circuit and being so constructed and arranged that it produces interruptions in said carrier wave, the durations of which are greater than the delay time between the first and last significant or important pulses received over the multiple paths for each pulse transmitted over the antenna.

7. A transmitter system for reducing fading in radio circuits caused by the multipath phenomenon in the atmospheric signal medium comprising a source of high frequency waves, a modulator coupled to said source, an impulse generator coupled to said modulator for interruptme said carrier wave, an amplifier in the output of said modulator, an antenna coupled to said amplifier, and a signal modulation system coupled to said amplifier, said impulse generator including a differentiator circuit and being so constructed and arranged that it produces interruptions in said carrier wave, the durations of said interruptions being greater than the delay time between the first and last significant or important pulses received over the multiple paths for each pulse transmitted over the antenna, and a characteristic of the modulated interrupted carrier wave varying in accordance with the signal modulation applied thereto.

8. A transmitter for sending out pulses of carrier waves comprising a source of carrier waves, a modulator coupled to said source, an amplifier coupled to the output of said modulator, an antenna coupled to said amplifier, an impulse generator in the form of a blocking oscillator for producing a wave of rectangular form, a high frequency pass filter coupled to said blocking oscillator for producing sharp pulses from the slopes of the rectangular wave produced by said oscillator, an amplifier for amplifying said sharp pulses, and a connection from said last amplifier to said modulator for causing said modulator to pass pulses of carrier oscillations, and means for modulating a characteristic of said oscillations.

9. A radio transmitter for radiating pulses of power of constant high frequency carrier oscillations, including a pulse generator for modulating the pulse rate in accordance with the signal modulation, said pulse'generator comprising a pair of oscillators which produce different frequencies of oscillation when excited individually and which produce a frequency of oscillation midway between the individual frequencies when excited simultaneously,

10. A radio transmitter for radiating pulses of power of constant high frequency carrier oscillations, including a pulse generator for modulating the pulse rate in accordance with the signal modulation, said pulse generator comprising a pair of oscillators which produce different frequencies of oscillation when excited individually and which produce a frequency of oscillation midway between the individual frequencies when excited simultaneously, each of said oscillators having a time constant circuit composed of a resistance and a condenser for determining the frequency of oscillation thereof, said time constant circuits being tuned to different frequencies, a common anode resistor for said oscillators, and means for applying signal modulating potentials to the inputs of said oscillators, whereby the frequency of output of said pulse generator is modulated between the individual frequencies of the two oscillators and the generator produces 8 said midfrequency for the unmodulated condi tion.

11. A radio transmitter for radiating pulses of power comprising a carrier source, a modulator coupled to said source, a power amplifier coupled to said modulator, an antenna coupled to the output of said power amplifier, and an impulse generator also coupled to said modulator for causing an interrupted flow of carrier frequency oscillations through said modulator, said impulse generator producing pulses of diiierent frequencies in response'tomodulating potentials.

1.2. A transmitter for sending out pulses of carrier waves comprising a source of carrier waves, a modulator coupled to said source, an amplifier coupled to the output of said modulator, an antenna coupled to said amplifier, an impulse generator in the form of av blocking oscillator for producing a wave of rectangular form, a, high frequency pass filter coupled to said blocking oscillator for producing sharp pulses from the slopes of the rectangular wave produced by said oscillator, an amplifier for amplifying said sharp pulses, and a connection from said last amplifier to said modulator for causing said mod-1 ulator to pass pulses of carrier oscillations, and means for keying said pulses of carrier oscillations. 7

13. A pulse type transmitter comprising a source of radio frequency carrier waves, a modulator coupled to the output of said source, a pulse generator coupled to the input of said modulator for producing an output from said modulator composed of pulses of radio frequency carrier waves, and means for modulating the pulse rate in accordance with signals to be transmitted.

14. A pulse type transmitter comprising a. source of radio frequency carrier waves, a modulator coupled to the output of said source, a pulse generator coupled to the input of said modulator for producing an output from said modulator composed of equal width pulses of radio frequency carrier waves, and means for modulating the pulse rate in accordance with signals to be transmitted.

15. A pulse type transmitter comprising a source of radio'frequency carrier waves, a modulator coupled to the output of said source, an electron discharge device pulse generator, means coupled between the output of said pulse generator and the input of said 1110611 118301 for producing short impulses which occur at the start and stop of the longer duration pulses produced by said pulse generator to thereby cause said modulator'to produce pulses of radio frequency carrier waves, and means for controlling the number of pulses to be sent out in accordance with signals to be transmitted.

16, A pulse type transmitter comprising a source of radio frequency carrier waves, a modulator coupled to the output of said sour e, an

electron discharge device pulse generator, means coupled between the output of said pulse generator and the inp t of said modulator for pro ducing short impulses. which occur at the start and stop of the longer duration pulses produced a two different signal code characters.

7.,The method of reducing fading in a radio circuit produced by multipath effects in the atmospheric signal medium between a transmitter and a receiver which comprises radiating signal energy in pulses having a predetermined frequency rate of recurrence, each pulse having a time duration which is less than the delay time between the first and last significant or important pulses received over the multiple paths for each radiated pulse, and interrupting the radiation of said pulses in accordance with two difierent signal code characters to produce different trains of pulses of difierent recurrence rates.

18. A transmitter for sending out pulses of carrier waves comprising a source of carrier waves,

c 10 a modulator coupled to said source, an amplifier coupled to the output of said modulator, an antenna coupled to said amplifier, an impulse generator in the form of a blocking oscillator for producing a wave of rectangular form, a high frequency pass filter coupled to said blocking oscillator for producing sharp pulses from the slopes of the rectangular wave produced by said oscillator, means coupling said filter to said modulator for causing said modulator to pass pulses of carrier oscillations, and means for key-' ing said pulses of carrier oscillations.

MURRAY G. CROSBY. 

